Although mammography is very sensitive in detecting early breast cancer, it does not work well in women who have dense breasts, breast implants, or scar tissues. Alternatively, magnetic resonance imaging (MRI) has proven to the most sensitive imaging modality in delineating tumor extent and detecting multifocal or multicentric diseases. Many studies indicate that preoperative MRI is useful in local staging for surgical planning, especially for patients with lobular cancer. However, the variable specificity of MRI can lead to unnecessary biopsies and over-treatment.
Scintimammography (SMM) is a single photon breast imaging technique used to detect cancer cells in patients who have had abnormal mammograms or dense breast tissue. In this test, a patient will receive an injection of technetium 99 sestamibi, a gamma-ray emitter which is preferably taken up by cancer cells. The breast is then usually compressed and imaged by a gamma camera. SMM can potentially supplement MRI for improving the diagnostic specificity in breast cancer imaging.
The combination of MRI and SMM has great clinical potential for improving specificity through SMM while maintaining the high sensitivity offered by MRI. However, interference between the MRI and SMM components presents a significant challenge to the design of any combined system. This interference can lead to major artifacts and image degradation in both modalities. The primary concerns are electromagnetic interference and the effects of the B0 magnetic field of the MRI scanner on the SMM detectors. Traditional gamma-ray detectors based on scintillators coupled to photomultiplier tubes do not function properly within high magnetic fields. Instead, MR-compatible cadmium-zinc-telluride (CZT) semiconductor-based radiation detectors may be utilized. Such detectors, however, must be enclosed in RF shielding to prevent electromagnetic interference between the MRI and SMM systems. Furthermore, sufficient gamma-ray shielding is required to prevent the detection of uncollimated radiation. Both of these shielding layers have the potential to adversely affect MR imaging, particularly when they are placed within the RF coil.